Being a nice picnic and you can use the picnic tables. Big parking lot with access to the huge, wide open dunes. The dunes are are a bit of a walk from parking but worth it. Drink plenty of water! No specific trail on dunes. Just up, up, up, short bit of down, up, up, up.
You can park and just go. There's Medano Creek that should be running just alongside the parking lot. Super fun for splashing around in the hot sun. It is suprisingly windy there. I highly recommend long sleeves/pants and a bandana for little one's faces. As far as "trails" on the dunes, there's just natural slopes and easier ways to walk around on the dunes. It's like a gigantic sandbox! Sledding seems fun in theory but we didn't try it. The walk out to the dunes in the sand was quite the workout so be mindful of your limits.
Half day would be great. Yes there is parking, though you can advance further on to a dirt road only if you have a 4WD car. Otherwise you park a bit further away and have to (get to!) do some more walking. You could make it a longer day if you want to hike the surrounding trails as well as the dunes. Dunes are basically free range. Go wherever. But they're tiring! Plus there is a hike close by (Zapata Falls I believe it's called. Off of the road that leads in to the park) with a waterfall. So the region has a lot to offer in terms of beauty/recreation but not a lot of big city amenities anywhere near the park.
Yes, you basically park, then walk out directly into the dunes. it is a good walk across a "desert"(super flat, kind of squishy, nothing but sand...) to the physical dunes, but we're talking a 10 or 15 minute walk if that before you start climbing. There's also a visitor center with stuff about why they are there etc... Any additional routes are meant for 4x4 vehicles. There are lots of off-road paths and such but just one main entrance otherwise.
Mostly just park at the day use, dunes parking area. Half a day will go by quickly with just sledding and maybe a visit to the vistors center. Enjoy it's a beautiful place
All of these comments and more but you may have the option of spending a half night there!
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